A number of commercially available polysiloxane resins are used in powder coatings. In a typical formulation they are combined with at least one organic resin, and usually with an organic curing agent as well. Examples of these coatings are the silicone/GMA-acrylic blends of Reisling (U.S. Pat. No. 4,877,837) and Eklund (U.S. Pat. No. 5,684,066) and the silicone/acid-functional acrylic/triglycidyl isocyanurate compositions of Daly et al. (U.S. Pat. No. 5,422,396).
Another desirable category of coatings could be called 100% silicone coatings. These coatings contain no non-silicone resins or non-silicone stoichiometric curing agents. Cure in these coatings is effected by catalysts which accelerate the condensation of siloxyl (SiOH) end groups of silicone resins as in Scheme 1. ##STR1##
Coatings that are 100% silicone are desirable, for example, because they have superior color stability at high temperature. This is especially useful in white and light colored coatings.
Such coatings have appeared in patents. For example, Daly et al. in U.S. Pat. No. 5,422,396 discloses in a "comparative example" a 100% silicone coating based on Dow Corning 6-2230 silicone resin. Beginning in 1994 and 1995, Morton International, Inc. sold 100% silicone coating powders based on Dow Corning 6-2230. However, these were commercially unsuccessful and less than 1000 pounds were sold. Significant deficiencies including severe blocking of the powder, i.e., the tendency of the powder to clump together, and foaming during curing. Hence, it is a primary object of the present invention to provide a 100% (of the binder resin) silicone coating powder which neither blocks significantly nor foams and which forms a smooth, continuous coating.